1. Field of the Invention
The present invention relates to an adhesive composition. More specifically, it relates to an adhesive composition having excellent adhesion, soldering heat resistance (i.e., heat resistance against soldering), flexibility, and chemical resistance and having small resin flow, suitable for use in the production of a flexible printed board materials (or flexible printed circuit, "FPC" below).
2. Description of the Prior Art
Heat stable plastic films such as polyimide films, or polyester films have long been used in the production of FPC. For instance, metallic foils such as copper foils and aluminum foils are adhered to polyimide films and polyester films by using adhesives to form FPC. These FPC are used in various application fields including cameras, pocket or desk calculators and telephone sets. The adhesives for use in the production of FPC should have various necessary characteristics. Not only must they bond the films and metallic foils, but they also must have electric characteristics, soldering heat resistance (e.g., resistance to blistering or other abnormalities upon immersion in a molten solder having a high temperature), chemical resistance against solvents such as methylene chloride used for dissolving portions other than the circuits, and flexibility required for free three-dimensional circuits. Futhermore, the increasing production and greater design sophistication and complexity of FPC have resulted in demand for higher performance adhesives of greater reliability. Known adhesives heretofore used for FPC include fluoro resins, epoxy-novolak resins, nitrile-phenol resins, polyester resins, and acrylic resins. However, no satisfactory adhesive having all of the above-mentioned characteristics and requirements has yet been developed.
Furthermore, in order to improve the insulating characteristics of flexible circuits, an insulation film having an adhesive layer coated on one surface thereof is often attached to the printed board material comprising a film and a metallic foil after the desired circuit processing. However, when the insulation film is contact bonded upon heating to the printed board material, an undesirable phenomenon, which is referred to as "resin flow" occurs, wherein the adhesive layer of the insulation film is squeezed out to the pad portion.